This invention relates in general to apparatus for advancing a strip of sheet material over the work surface of a table by precisely controlled amounts and deals more particularly with an improved apparatus of the type wherein a carriage used to position an instrument over the work surface is also employed to advance or shift the strip of sheet material relative to the work surface when the instrument is not in operation.
A typical automatically controlled instrument system of the type with which the present invention is concerned has carriages which move in two coordinate directions over the work surface of a table. Typically, a first carriage traverses the table in one coordinate direction parallel to the work surface and a second carriage mounted on the first moves relative to the first and in another coordinate direction. When an instrument is mounted on the second carriage composite movements of both carriages allow the instrument to be translated to any point over the region of the work surface traversed by the carriages. Accurate positioning of the carriages and the instrument carried thereby may be achieved by numerical controls which may operate either from an on-line data generator or from previously programmed data. Such machines may be provided with a wide variety of instruments, as, for example, plotting pens or styluses, light heads, tracking heads and cutting or drilling tools. In a machine of this type, a strip of sheet material which is substantially longer than the machine table can be handled by providing means for accurately advancing or indexing the sheet material over the table surface so that the instrument may operate on successive sections of the material.
The difficulty in working on successive sections of a strip of material is that movement of each section of the strip into working position must be precisely controlled to permit generation of continuous patterns which extend between adjacent sections of the strip. In a high resolution plotting system, for example, a series of plotted lines may extend continuously over several successive sections of a strip which is substantially longer than the plotting table. The strip indexing operation, unless accurately controlled, may cause discontinuity in the plot resulting in plotted lines on adjacent sections being out of registration. This problem has been overcome by providing apparatus which includes means for releasably coupling sheet material to the instrument carriage so that the sheet material may be moved relative to the work surface on which it is supported with the same degree of accuracy attained in moving the instrument. Such apparatus is illustrated and described in U.S. Pat. No. 3,844,461 to Samuel Clifford Robison et al for PRECISE INDEXING APPARATUS AND METHOD, issued Oct. 29, 1974, and assigned to the assignee of the present application.
The present invention is concerned with improved apparatus of the type shown in the patent to Robison et al and particularly with apparatus which includes improved means for coupling sheet material to a carriage for advancing or indexing movement therewith.